Apparatus and method of measuring terahertz wave

ABSTRACT

A time-domain waveform of a terahertz wave is measured by a method based on time-domain spectroscopy by using an optical delay unit to adjust an optical path length along which excitation light propagates thereby adjusting a difference between a time at which the excitation light arrives at a generating unit configured to generate the terahertz wave and a time at which the excitation light arrives at a detection unit configured to detect the terahertz wave. The optical delay unit is driven according to a first speed pattern to acquire a first time-domain waveform. The optical delay unit is then driven according to a second speed pattern different from the first speed pattern to acquire a second time-domain waveform. The first time-domain waveform and the second time-domain waveform are averaged.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. patent applicationSer. No. 13/096,803, filed Apr. 28, 2011, which claims foreign prioritybenefit of Japanese Patent Application No. 2010-113828 filed May 18,2010 and Japanese Patent Application No. 2010-175824 filed Aug. 5, 2010.The above-named applications are hereby incorporated by reference hereinin their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and a method of measuringa terahertz wave, and more particularly, to an apparatus and a method ofmeasuring a terahertz wave in a time domain. Hereinafter, such as anapparatus will be also referred to as a THz TDS (Time DomainSpectroscopy) apparatus.

2. Description of the Related Art

A terahertz wave is an electromagnetic wave with a frequency in anarbitrary frequency band within a range from 0.03 THz to 30 THz. Thisfrequency range includes many frequencies or bands of frequency at whichcharacteristic absorption occurs due to structures or states ofsubstances such as biological molecules. This feature is usable tonondestructively analyze or identify a substance, and associatedtechniques have been developed. One example of a predicted applicationis a safety imaging technique usable instead of an X-ray imagingtechnique. Another example of a predicted application is a high-speedcommunication technique.

In the time domain, terahertz waves generally have a form of a pulsewith a width of sub-pico seconds. It is generally difficult to acquiresuch a pulse in real time because of the slow response that currentelectronics have with respect to the speed of THz waves. To overcomesuch a difficultly, a conventional THz-TDS apparatus employs a samplingmeasurement technique using ultrashort pulse light with a pulse width onthe order of femto seconds. The sampling of the terahertz wave isachieved by adjusting a difference between a time at which excitationlight arrives at a generating unit that generates the terahertz wave anda time at which the excitation light arrives at a detection unit thatdetects the terahertz wave. For example the time difference can beprovided by disposing a stage having a folded optical system (alsoreferred to as an optical delay unit in the present description) in apropagation path of the excitation light and adjusting the totalround-trip length of the excitation light in the folded optical system(see, for example, Japanese Patent Laid-Open No. 2008-20268). In manycases, the generating unit and/or the detection unit is realized using aphotoconductive device including an antenna electrode pattern havingsmall gaps formed on a semiconductor film.

In the THz-TDS apparatus, an increase in the measurement sensitivity canresult in an increase in effects of a vibration of the stage of theoptical delay unit. More specifically, the vibration of the stage usedin the optical delay unit causes the optical axis of the excitationlight to swing. This causes a change in the amount of light per unitarea that strikes the small gaps of the photoconductive device. Thus, avibration component is superimposed on the time-domain waveform of theterahertz wave reproduced by the apparatus. If such a time-domainwaveform is subjected to a Fourier transform, then, as shown in FIG. 6,a resultant spectrum 623 of the terahertz wave detected by the detectionunit includes a spurious component 624 due to the vibration of theoptical delay unit. For example, when a vibration component of severalhundred Hz is superimposed on a time-domain waveform of a terahertzwave, a spurious spectrum appears typically at 4 THz to 6 THz althoughthe spurious spectrum varies depending on the configuration of themeasurement system and/or the driving condition of the optical delayunit. Such a spurious spectrum can limit a measurement bandwidth of themeasurement apparatus and can cause a reduction in analysis performancethereof. As can be seen from the above description, in the terahertzwave measurement apparatus, there is a need for suppression of effectsof vibrations of an optical delay unit.

SUMMARY OF THE INVENTION

According to an aspect, the present invention provides a method ofmeasuring a time-domain waveform of a terahertz wave based ontime-domain spectroscopy by using an optical delay unit to adjust anoptical path length along which excitation light propagates therebyadjusting a difference between a time at which the excitation lightarrives at a generating unit configured to generate the terahertz waveand a time at which the excitation light arrives at a detection unitconfigured to detect the terahertz wave, the method includes driving theoptical delay unit according to a first speed pattern to acquire a firsttime-domain waveform, driving the optical delay unit according to asecond speed pattern different from the first speed pattern to acquire asecond time-domain waveform, and averaging the first time-domainwaveform and the second time-domain waveform.

In the method according to one aspect of the invention, the speedpattern used to drive the optical delay unit is changed for eachmeasurement of the time-domain waveform, and measurement results areaveraged. Changing the speed pattern results in a change in samplingtime intervals at which data is acquired to reproduce the time-domainwaveform of the terahertz wave. The time-domain waveform of theterahertz wave reproduced does not depend on the sampling timeintervals. However, the change in the sampling time interval causes achange in shape of a signal component having substantially no relationwith the terahertz wave. This property makes it possible to suppress thesignal components having the small relation with the terahertz wave byaveraging data obtained for various speed patterns. Thus, it becomespossible to suppress a spurious spectrum superimposed on a frequencyspectrum and it becomes possible to increase a measurement bandwidth.

Further aspects of the present invention will become apparent to personshaving ordinary skill in the art from the following description ofexemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of a configuration of ameasurement apparatus according to an embodiment of the invention.

FIG. 2A is a graph illustrating a maximum speed Vmax included in a speedpattern.

FIG. 2B is a graph illustrating a minimum speed Vmin included in a speedpattern.

FIG. 3A is a graph provided for explaining a measurement apparatus, amethod, and a process according to an embodiment of the presentinvention.

FIG. 3B is a graph provided for explaining a measurement apparatus, amethod, and a process according to an embodiment of the presentinvention.

FIG. 3C is a graph provided for explaining a measurement apparatus, amethod, and a process according to an embodiment of the presentinvention.

FIG. 3D is a graph provided for explaining a measurement apparatus, amethod, and a process according to an embodiment of the presentinvention.

FIG. 4 is a flow chart associated with a measurement apparatus, amethod, and an operation according to an embodiment of the presentinvention.

FIG. 5A is a graph provided for explaining a measurement apparatus, amethod, and a plurality of speed patterns according to an embodiment ofthe present invention.

FIG. 5B is a graph provided for explaining a measurement apparatus, amethod, and a plurality of speed patterns according to an embodiment ofthe present invention.

FIG. 5C is a graph provided for explaining a measurement apparatus, amethod, and a plurality of speed patterns according to an embodiment ofthe present invention.

FIG. 6 is a graph illustrating a problem to be solved by an embodimentof the present invention.

FIG. 7 is a graph illustrating a driving speed included in a speedpattern according to an embodiment of the present invention.

FIG. 8A is a graph illustrating a comparative example of a time-domainwaveform.

FIG. 8B is a graph illustrating a measured time-domain waveformaccording to an embodiment of the present invention.

FIG. 9A is a graph illustrating a comparative example of a frequencyspectrum.

FIG. 9B is a graph illustrating a measured frequency spectrum accordingto an embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

In an apparatus and a method of acquiring a time-domain waveform of aterahertz wave using time-domain spectroscopy according to an aspect ofthe present invention, a plurality of time-domain waveforms are acquiredby driving an optical delay unit according to a plurality of differentspeed patterns, and a final time-domain waveform is acquired byaveraging the plurality of time-domain waveforms. A basic configurationand a spirit of the apparatus and the method of acquiring thetime-domain waveform of the terahertz wave using the time-domainspectroscopy according to an aspect of the invention have been describedabove.

Next, specific embodiments of the invention are described below withreference to the accompanying drawings. FIG. 1 is a diagram illustratingan example of a configuration of a terahertz wave measurement apparatusaccording to an embodiment of the invention. As shown in FIG. 1, theapparatus includes a generating unit 101, a detection unit 102, anoptical delay unit 103 including a moving part 103 a, a driving speedadjustment unit 104, a processing unit 105, a bias applying unit 106, acurrent detection unit 107, and a laser light source 108.

The generating unit 101 is a device that generates a terahertz wave. Inthe generating unit 101, the terahertz wave may be generated by a methodusing an instantaneous current or a method using interband transition ofcarriers. In the method using the instantaneous current, the terahertzwave may be generated using a technique of illuminating a surface of asemiconductor or an organic crystal with excitation light. In thistechnique, the illuminating of the excitation light may be performedwhile applying an electric field to a device (photoconductive device)having a metal antenna pattern formed on a thin semiconductor film.Alternatively, a positive-intrinsic-negative (PIN) diode may be used. Onthe other hand, in the method using the interband transition of carriesin the gain structure, a semiconductor quantum well structure may beused. It is expected that persons having ordinary skill in the art wouldbe familiar with these and other conventional structures useful forterahertz-wave generation.

The detection unit 102 is a unit that detects the electric fieldintensity of the terahertz wave. In the detection unit 102, the electricfield intensity of the terahertz wave may be detected by detecting acurrent that occurs in response to a change in photoconductivity causedby illumination of the excitation light. In this technique, the currentmay be detected using a photoconductive device of the type describedabove. An alternative method is to detect an electric field using anelectrooptical effect. A further alternative method is to detect amagnetic field using a magnetooptical effect. A specific example of themethod of detecting the electric field using the electrooptical effectis to use a polarizing beam splitter (polarizer) and an electroopticalcrystal. A specific example of the method of detecting the magneticfield using the magenetooptical effect is to use a polarizing beamsplitter (polarizer) and a magnetooptical crystal. In the followingdescription, by way of example, the generating unit 101 and thedetection unit 102 are each implemented using a photoconductive device.

In the present embodiment, the laser light source 108 is a coherentlight source that outputs ultrashort (generally pico or femtosecond)pulses of light. The above-described generating unit 101 and thedetection unit 102 are configured to operate by irradiating a thinsemiconductor film with the ultrashort pulse of laser light to excitecarries in the thin semiconductor film. In the present description, inview of the above, the ultrashort pulse laser light is also referred toas excitation light. As shown in FIG. 1, the excitation light emittedfrom the light source 108 is split into two optical paths L1 and L2. Inthe present embodiment, the excitation light passing through the opticalpath L1 illuminates the generating unit 101. The excitation lightpassing through the optical path L2 illuminates the detection unit 102via the optical delay unit 103 described later.

In many cases, the time-domain waveform of a terahertz wave is in a formof a pulse with a width of picoseconds or less, and thus it is difficultto acquire the waveform of the terahertz wave in real time. For thisreason, the time-domain waveform of the terahertz wave is acquired via asampling measurement process using the excitation light. The opticaldelay unit 103 is configured to adjust the delay time between a time atwhich the terahertz wave is generated by the generating unit 101 and atime at which the terahertz wave is detected by the detection unit 102to adjust sampling points at which data is acquired to produce thetime-domain waveform of the terahertz wave. More specifically, the timefor the excitation light to arrive at the detection unit 102 is delayedwith respect to the time for the excitation light to arrive at thegenerating unit 101. The difference in arrival time of the excitationlight between the generating unit 101 and the detection unit 102 may becontrolled by directly adjusting the length of the optical propagationpath of the excitation light or by adjusting the effective length of theoptical path. A specific method of the direct adjustment of the opticalpath length is to provide a folded optical system in which excitationlight is reflected back and move this optical system in the samedirection as the folded direction of the optical path by using themoving part 103. A specific method of adjusting the effective length ofthe optical path is to change a time constant (refractive index) in theoptical path along which the excitation light propagates. In the exampleshown in FIG. 1, a one-stage folded optical system and a linear motionstage serving as the moving part 103 a are used. In this example, theoperation of the moving part 103 a is controlled by the driving speedadjustment unit 104 that will be described in further detail later. Theoptical path length L2 from the laser light source 108 to the detectionunit 102 is changed by changing the position of the folded opticalsystem by using the moving part 103 a. The change in the optical pathlength is controlled such that the difference in optical path lengthL2−L1 causes a particular time difference between a time at which theexcitation light reaches the generating unit 101 and a time at which theexcitation light reaches the detection unit 102. If the driving speed ofthe moving part 103 a is high, the time-domain waveform of the terahertzwave can be acquired in a short time.

The bias applying unit 106 is a unit that provides a bias to drive thegenerating unit 101. In a case where a photoconductive device is used asthe generating unit 101, a voltage is applied to a metal electrodeincluding an antenna pattern. In particular, when the current detectionunit 107 (described later) includes a lockin detection system, thevoltage supplied by the bias applying unit 106 is modulated by afrequency equal to that of a reference signal in the lockin detectionsystem. In the lockin detection, instead of modulating the bias suppliedby the bias applying unit 106, the modulation may be achieved bymodulating the excitation light using an optical chopper. In the lattermethod, the bias applying unit 106 applies a DC bias directly to thephotoconductive device.

The current detection unit 107 may be a circuit that converts a currentsignal into a voltage signal with a measurable level. When aphotoconductive device is used as the detection unit 102, the currentdetection unit 107 converts a current signal output from the detectionunit 102 into a voltage signal. The conversion ratio of the currentsignal to the voltage signal is referred to as a current-to-voltageconversion ratio. The current-to-voltage conversion ratio is selectedwithin a range that allows the current detection unit 107 to convert thecurrent signal input thereto to the voltage signal without causing theoutput of the current detection unit 107 to exceed a rated valuespecified for the circuit. To increase the signal-to-noise ratio of themeasurement apparatus, the current-to-voltage conversion ratio may beset to be as large as possible. As described above, when the signaloutput from the detection unit 102 is weak, the current detection unit107 may include a lockin detection system. More specifically, the lockindetection system is disposed at a final stage of a circuit that performsthe current-to-voltage conversion. In the case where the currentdetection unit 107 includes the lockin detection system, the circuitthat performs the current-to-voltage conversion is set such that theamplitude of the signal output therefrom does not exceed the inputrating of the lockin detection system. Note that the current detectionunit 107 may be replaced with another unit configured to provide, basedon the signal output from the detection unit 102, a signal that can beeasily processed by the processing unit 105.

The processing unit 105 is a unit that provides measurement data byproducing the time-domain waveform of the terahertz wave. Thetime-domain waveform is produced based on the amount of change in theoptical path length of the optical delay unit 103 and the output of thecurrent detection unit 107. More specifically, the time-domain waveformis produced by plotting the output of the current detection unit 107 insteps of predetermined amounts of change in the optical path length. Thepredetermined amount of change in the optical path length corresponds toa time interval t of the measurement data. The measurement data isobtained in the form of a series of intensity data plotted at timeintervals t, and the obtained measurement data is stored. To improve thesignal-to-noise ratio of the measurement apparatus, the linear motionstage of the optical delay unit 103 may be stopped at each measurementpoint (or the speed is reduced to a level that can be regarded as beingsubstantially at rest), and outputs provided by the current detectionunit 107 are averaged to obtain a final time-domain waveform. Thistechnique is referred to as a step-and-scan method. An alternativetechnique is to acquire the time-domain waveform a plurality of timeswhile driving the linear motion stage of the optical delay unit 103 at ahigh speed. The acquired time-domain waveforms are averaged by theprocessing unit 105 for respective elements of the sequence of measuredintensity data. This technique is referred to as a rapid scan method. Asdescribed above, the time-domain waveform is acquired a plurality oftimes to obtain a plurality of sets of data of the time-domain waveform,and the plurality of sets of data are averaged by the processing unit105 to increase the signal-to-noise ratio of the signal.

To output spectrum data in the frequency domain, the processing unit 105refers to the measurement data and performs a Fourier transform on thetime-domain waveform of the terahertz wave to acquire the spectrum data.When the THz-TDS apparatus is used as an analysis apparatus, a sample(specimen) is irradiated with a terahertz wave and a change intime-domain waveform of the terahertz wave caused by the irradiation isdetermined. More specifically, the specimen is irradiated with theterahertz wave generated by the generating unit 101, and the terahertzwave passing through the specimen or the terahertz wave reflected fromthe specimen is detected by the detection unit 102. Based on theobtained time-domain waveform, information on the specimen is acquired.The processing unit 105 may acquire an image of the sample by monitoringa relative position between the sample and the terahertz waveirradiating the sample. With the configuration described above, theTHz-TDS apparatus monitors a change in optical path length of excitationlight provided by the optical delay unit 103 and a corresponding changein output of the current detection unit 107, and the THz-TDS apparatusproduces a time-domain waveform of the terahertz wave irradiating thedetection unit 102.

A typical known configuration of the THz-TDS apparatus has beendescribed above. In the present embodiment, in addition to theconfiguration described above, the apparatus further includes thedriving speed adjustment unit 104. The driving speed adjustment unit 104makes it possible to acquire the time-domain waveform of a terahertzwave by a unique method according to the present embodiment of theinvention. The driving speed adjustment unit 104 is a unit that managesand adjusts the speed of the moving part 103 a of the optical delay unit103. Note that the speed of the moving part 103 a is managed for eachtime-domain waveform measured. More specifically, the moving part 103 ais driven at a particular speed that is changed for each measurement ofthe time-domain waveform. The speed of the moving part 103 a may bechanged even during a process of measuring a single time-domainwaveform. In the present description, a mode of changing the speed isdefined by a speed pattern. The driving speed adjustment unit 104manages the speed pattern and provides the managed speed pattern.

FIGS. 5A to 5C illustrate examples of speed patterns managed by thedriving speed adjustment unit 104. More specifically, FIGS. 5A to 5Cillustrate examples of changes in optical path length of the opticaldelay unit 103 as a function of an elapsed time. That is, bycontinuously detecting the change in optical path length with theelapsed time, it is possible to determine and control the driving speedof the moving part 103 a of the optical delay unit 103. As can be seen,the change in optical path length is folded at a particular value,because the moving part 103 a moves back and forth. As shown in FIGS. 5Ato 5C, a different speed pattern is defined for each round trip of themovement of the moving part 103 a. The time-domain waveform of aterahertz wave is produced for each round trip of the moving part 103 a.And, since each round trip of the moving part 103 a is performed at adifferent speed pattern, a time-domain waveform is produced for eachspeed pattern by the processing unit 105. In each of the examples shownin FIGS. 5A to 5C, there are n speed patterns defined, and thus thetime-domain waveform of the terahertz wave is produced n times.

FIG. 5A illustrates an example in which the speed pattern is managed foreach measurement of the time-domain waveform. In this example, eachspeed pattern changes at a constant speed in each measurement of thetime-domain waveform of the terahertz wave. Specifically, in the exampleof FIG. 5A, when the moving part 103 a moves under the first speedpattern, the optical path length changes constantly at a first speed;when the moving part 103 a moves under the second speed pattern, theoptical path length changes constantly at a second speed different fromthe first speed. The same applies for the third to n-th speed patterns.FIG. 5B illustrates an example in which the driving of the moving part103 a is performed at a varying speed during each measurement of thetime-domain waveform. That is, in this example, each speed patternchanges at a non-constant speed in each measurement of the time-domainwaveform of the terahertz wave. Specifically, in the example of FIG. 5b, when the moving part 103 a moves under the first speed pattern, theoptical path length changes at a varying first speed; and when themoving part 103 a moves under the second speed pattern, the optical pathlength changes at a varying second speed different from the varyingfirst speed. The same applies for the third to n-th speed patterns. FIG.5C illustrates an example in which a waiting period with a fixed length(during which the optical path length is maintained unchanged, i.e., thespeed of the moving part 103 a is zero) is provided after a measurementpoint is reached in each speed pattern, and the driving speed is changedagain after the end of the waiting period. In the example shown in FIG.5C, the back-and-forth movement of the moving part 103 a is performedsuch that the speed pattern in a backward direction is different fromthat in a forward direction. Note that the speed pattern may be set tobe different between the forward and backward directions also in theprevious examples described above. Note that in conventional THz-TDSapparatuses, the same speed pattern is used for all measurement cycles.In contrast, in the present embodiment of the invention, the speedpattern is different for each measurement of the time-domain waveform ofthe terahertz wave.

Referring to FIGS. 2A and 2B, an explanation is given below as to alimitation on the speed in the speed patterns. FIG. 2A illustrates amaximum speed Vmax in a speed pattern. More specifically, FIG. 2Aillustrates a band of a frequency spectrum of a terahertz wave as afunction of a speed of change in the length of the optical path of theexcitation light adjusted by the optical delay unit 103.

In FIG. 2A, X included in a variable in a horizontal axis indicates achange in optical path length (m/point) per a predetermined timeinterval (denoted as a point in FIG. 2A) at which a time-domain waveformof a terahertz wave is sampled to acquire measurement data of thetime-domain waveform. The change in optical path length divided by thespeed of light equals to the predetermined time interval. τ is a timeconstant of the THz-TDS apparatus. For example, in a case where theapparatus is configured to perform a lockin detection using a lockinamplifier, τ generally has a value corresponding to a time constant ofthe lockin amplifier.

V denotes the speed of the change in the optical path length adjusted bythe optical delay unit 103. Thus, the variable X/τ/V (X divided by τ andfurther divided by V) in the horizontal axis indicates a valuenormalized by τ in terms of a time X/V needed to achieve a particularchange in the optical path length. The speed V of the change in opticalpath length decreases with increasing value in the horizontal axis. Avertical axis indicates a frequency band defined by a frequency at whichthe electric field intensity of the frequency spectrum of the acquiredterahertz wave has a negative 3 decibel (−3 dB) value with respect tothe highest value. In FIG. 2A, data is plotted while changing themodulation frequency of the bias applying unit 106 from 1 KHz to 100KHz. A solid line indicates an approximate curve determined so as to fitthe plotted data. In FIG. 2A, from a change in gradient of a tangent tothe approximate curve, it can be seen that an inflection point occurs atapproximately X/τ/V=5. When X/τ/V is in a range equal to or greater than10, the approximate curve with a gradient of 0 fits well the plotteddata. When X/τ/V is smaller than 5, the speed of the change in theoptical path length is likely to become excessively large with respectto the time constant τ of the apparatus. That is, the speed of thechange in the optical path length can become too fast, that the lockinamplifier can no longer detect and control change in the optical path.This can lead to an increase in tendency to have noise and an increasein probability that it becomes difficult to accurately capture thetime-domain waveform of the terahertz wave.

In view of the above, preferably, the maximum speed Vmax in the speedpattern is set such that X/τ/Vmax is equal to or greater than 5, andmore preferably equal to or greater than 10. By limiting the maximumspeed of the change in the optical path length in the above-describedmanner such that the measurement apparatus can respond to the changeduring the process of acquiring the data, it becomes possible tosuppress the negative effect of the time constant of the measurementapparatus on the time-domain waveform of the terahertz wave and thus itbecomes possible to stably acquire the time-domain waveform of theterahertz wave. Note that, as can be seen from FIG. 2A, the data doesnot significantly depend on the modulation frequency.

Referring to FIG. 2B, a minimum speed Vmin in each speed pattern isdiscussed below. In FIG. 2B, a horizontal axis indicates a time neededto acquire a time-domain waveform, and a vertical axis indicates aprobability of occurrence of such a time. In the present embodiment, anaverage speed Vave is determined and a total measurement time needed forthe THz-TDS apparatus to perform the measurement is predicted based onthe average speed Vave. The average speed Vave is determined by anoperator by dividing the optical path length A needed to acquire thetime-domain waveform by the average time needed to acquire onetime-domain waveform. The speed included in the speed pattern has adistribution around the average speed Vave.

In the example shown in FIG. 2B, the measurement time performed usingthe speed included in the speed pattern has a normal distribution aroundthe time (A/Vave) needed for the measurement apparatus to adjust theoptical path length A with the average speed Vave. That is, themeasurement time has a normal distribution in a range from a minimummeasurement time (A/Vmax) needed when the driving is performed at amaximum speed Vmax to a maximum measurement time (A/Vmin) needed whenthe driving is performed at a minimum speed Vmin. Note that the minimumspeed Vmin is defined as Vmax·Vave/(2Vmax−Vave). In the case of thespeed pattern including a waiting period such as that shown in FIG. 5C,the minimum speed Vmin is calculated for a period excluding the waitingperiod, i.e., the minimum speed is defined for the period in which theoptical path length changes.

Vmax and Vmin may be set such that the time difference between theaverage measurement time and the minimum measurement time and the timedifference between the average measurement time and the maximummeasurement time are as large as allowed. In other words, an adjustmentis made to maximize the difference between Vave and Vmax and thedifference between Vave and Vmin. The adjustment of the Vave, Vmax, andVmin in the above-described manner makes it possible to provide a widevariety of speed patterns usable, which makes it possible to moreeffectively suppress spurious spectra.

In the apparatus and the method according to the present embodiment ofthe invention, Vmax and Vmin are determined in the above-describedmanner to prevent an occurrence of a state in which extra measurementtime is spent in acquisition of the time-domain waveform. That is, bysetting the driving speed included in the speed pattern defining thechange in the optical path length such that the driving speed has adistribution around the average speed determined by an operator based onthe maximum speed, it becomes possible to predict an approximate timeneeded to acquire a time-domain waveform of a terahertz wave, whichmakes it possible to perform a measurement in an efficient manner. Notethat the time does not need to have a normal distribution, but the timemay have other distributions such as a rectangular-shape distribution,and the speed pattern may be set such that the measurement time has adistribution around the average measurement time. The speeddistributions in speed patterns may be adjusted such that a first speedpattern and a second speed pattern are different in shape but each speedpattern has a similar distribution. In this case, even when ameasurement is terminated before the measurement is performed thepredetermined number of times, the same speed distribution is obtained.Alternatively, the speed may have a particular distribution such as thatshown in FIG. 2B as a whole from the first speed pattern to the n-thspeed pattern.

Indeed, a plurality of speed patterns may be determined in advance andmay be stored in the driving speed adjustment unit 104. Alternatively,speed patterns may be produced as required based on the average speeddefined by the operator and/or the maximum speed and the minimum speed.That is, after Vave, Vmax, and Vmin have been determined, the drivingspeed adjustment unit 104 may produce a plurality of speed patternsaccording to a particular condition. For example, the driving speedadjustment unit 104 may produce speed patterns such that each speedpattern is divided into one or more time slots represented in timedomain. And, a speed changing mode may be selected randomly or accordingto a predetermined selection rule from a predetermined group of valuescorresponding to a plurality of speed changing modes. Each selectedspeed changing mode can then be assigned to each time slot therebyproducing the plurality of speed patterns. In this manner, apredetermined speed pattern can be applied at each time slot to manageand adjust the speed of the moving part 103 a. In the producing of theplurality of speed patterns, data shown in FIGS. 2A and 2B may bereferred to. The group of speed changing modes may include, for example,a constant speed mode (a linearly changing mode), a sinusoidallychanging mode, a quadratically changing mode, etc. The group of speedchanging modes may be stored in a non-illustrated storage area of thedriving speed adjustment unit 104.

Next, referring FIG. 1, FIGS. 3A to 3D, FIG. 4, and other figures asrequired, the measurement apparatus, the method, and the operation ofthe measurement apparatus according to embodiments of the presentinvention are described below. FIGS. 3A to 3D illustrate signals inputto and output from the processing unit 105 in the apparatus in operationsteps according to the method disclosed herein. FIG. 4 illustrates aflow of the operation performed by the measurement apparatus accordingto the method. In FIG. 4, when the terahertz wave measurement apparatusstarts a measurement, the driving speed adjustment unit 104 sets a firstspeed pattern according to which to control the optical delay unit 103(step S401). The optical delay unit 103 adjusts the optical path lengthof excitation light according to the first speed pattern. In thefollowing explanation, it is assumed by way of example that the firstspeed pattern is set such that the optical path length changes at aconstant driving speed V as shown in FIG. 5A. If the driving speed V ishigh, the optical path length changes quickly. On the other hand, if thedriving speed V is low, the optical path length changes slowly.Specifically, as illustrated in FIG. 5A, the first speed pattern has asteeper slope than the second speed pattern. This means that the opticalpath length would constantly change faster when driven according to thefirst speed pattern than it would when driven according to the secondspeed patter. The processing unit 105 monitors a change in the opticalpath length. Each time a predetermined amount of change in optical pathlength is observed, the processing unit 105 acquires a value of acurrent detected by the current detection unit 107. This process isreferred to as sampling.

FIG. 3A illustrates a real-time waveform output from the currentdetection unit 107 (for example, a waveform obtained when the output ofthe current detection unit 107 is measured by an oscilloscope) forvarious sampling points. The real-time waveform is obtained in theprocess of changing the optical path length. More specifically, thevalues of the real-time waveform along the time axis are calculated fromthe amount of change in optical path length and the speed at which theoptical path length is changed. For example, a peak of a terahertz wavepulse occurs when the optical path length changes by a particular fixedamount. Similarly, there is a fixed relationship between each point inthe time-domain waveform of the terahertz wave and the amount of changein optical path length. However, the time needed to reach a particularpoint of the time-domain waveform of the terahertz wave depends on thespeed at which the optical path length is changed. Thus an apparentcontraction occurs for the real-time waveform of the terahertz wave whenthe optical path length is changed at high speed. Conversely, when theoptical path length is changed at a low speed, the real-time waveform ofthe terahertz wave expands compared with the example described above.That is, signal components of a terahertz wave (T1 or T2) propagating inspace can change such that the real-time waveform represented intime-domain expands or contracts depending on the speed pattern used. Inview of the above, the processing unit 105 acquires a value detected bythe current detection unit 107 each time a predetermined fixed amount ofchange occurs in the optical path length. The processing unit 105determines the time corresponding to the predetermined amount of changein optical path length (the time can be calculated by dividing thechange in optical path length by the speed of light) and plots the datato obtain a first time-domain waveform 211 as shown in FIG. 3C (stepS402). Note that the effect of the speed pattern is suppressed in thewaveform obtained in this manner.

After the time-domain waveform is obtained using the first speedpattern, the driving speed adjustment unit 104 sets a second speedpattern, different from the first speed pattern, for use in controllingthe optical delay unit 103 (step S403). Herein it is assumed by way ofexample that the second speed pattern is set such that the optical pathlength changes at a constant driving speed V/2 where V is the drivingspeed used in the first speed pattern. Use of this second speed patternwith the driving speed one-half that of the first speed pattern causesthe real-time signal output from the current detection unit 107 toexpand in the real time axis by a factor of two compared with the signalobtained when the first speed pattern is used, as shown in FIG. 3B. Asdescribed above, the processing unit 105 acquires the value detected bythe current detection unit 107 each time the predetermined fixed amountof change occurs in the optical path length. Therefore, the time neededto achieve the same predetermined amount of change is twice longer thanin the above case (FIG. 3A), and thus the sampling period expands by afactor of two in the real time axis. A waveform of a signal componentcaused by an operation of the measurement apparatus, such as a fixedvibration component caused by the driving of the optical delay unit 103,is insensitive to a change in speed pattern. As can be seen from FIGS.3A and 3B, signal components associated with terahertz waves (T1 and T2)propagating in space change in shape in response to a change in speedpattern, but only slight changes occur in the fixed signal components(such as the vibration of the optical delay unit 103) that are producedby the operation of the measurement apparatus and are superimposed onthe main signal components. For the second speed pattern, the processingunit 105 acquires the value detected by the current detection unit 107each time the predetermined fixed amount of change occurs in the opticalpath length. The processing unit 105 determines the time correspondingto the predetermined amount of change in optical path length and plotsthe data to obtain a second time-domain waveform 212 as shown in FIG. 3C(step S404).

As can be seen from comparison between the obtained first time-domainwaveform 211 and second time-domain waveform 212, components associatedwith terahertz waves propagating in space have the same shape becausethere is a fixed relationship between the point in the time-domainwaveform of the terahertz wave and the amount of change in the opticalpath length. On the other hand, for the components of the signal (suchas the vibration waveform of the optical delay unit 103 shown in detailin the circle of FIG. 3C) that are fixed in the time domain regardlessof the speed pattern, there is substantially no correlation with theamount of change in the optical path length. Therefore, when theprocessing unit 105 changes the sampling interval and acquires data atdifferent points, the fixed signal has different time-domain waveformsdepending on speed patterns as shown in FIG. 3C. That is, thetime-domain waveform produced by the processing unit 105 has a propertythat the signal components associated with the terahertz wavepropagating in space are the same in waveform regardless of the speedpattern, but the signal components having less correlation with theterahertz wave vary in waveform depending on the speed pattern.

After the acquisition of the second time-domain waveform is completed,the measurement apparatus determines whether to continue the measurementof the time-domain waveform (step S405). For example, when themeasurement has been performed a predetermined number of times, themeasurement of the time-domain waveform is ended.

Alternatively, the measurement may be ended when it is determined in afollowing processing step that a predetermined ending condition (forexample, in terms of a particular characteristic such as asignal-to-noise ratio) is satisfied. In a case where it is determinedthat the measurement is to be continued, the second speed pattern usedin the second measurement of the time-domain waveform is redefined asthe first speed pattern (step S406). The driving speed adjustment unit104 then sets a second speed pattern, different from the redefined firstspeed pattern, for use in controlling the optical delay unit 103 (stepS403), and measures the time-domain waveform using the second speedpattern.

As described above, the processing unit 105 reproduces data from eachmeasured time-domain waveform by converting the amount of change inoptical path length into the corresponding time by the calculation. Theprocessing unit 105 averages the data into a single time-domain waveform(step S407). The averaging may be performed by adding a plurality ofsets of data, for example, corresponding to a predetermined time slot ofthe time-domain waveform for each value in change in the optical pathlength and determining the mathematical average. Alternatively, data maybe preprocessed and the average may be determined for the preprocesseddata. An example of preprocessing is to correct baselines of measuredtime-domain waveforms. Another example is to performfast-Fourier-transform (FFT) filtering to suppress a signal in aparticular frequency range. A further example is to perform a wavelettransform to suppress system noise or a varying component in ameasurement environment. A still another example is to enhance data in aparticular time range (time slot in time domain) by using a time window.In addition to determining a mathematical average of the first to n-thtime-domain waveforms, a weighted average can be calculated, based on apredetermined weighting function of value thereof. For example, a peakof the terahertz wave that occurs substantially at the same point intime in the first to n-th waveforms can be assigned a weight value of90% or higher, while peaks that do not occur at the substantially thesame point in time in the first to n-th waveforms can be assigned aweight value of 10% or lower. The concept can be similar when applying apredetermined weighting function to the first to n-th time-domainwaveforms. In this manner, not only a mathematical average, but also aweighted average can be obtained. As described above, the processingunit 105 can average a plurality of sets of time-domain waveformsobtained based on detection signals provided by the detection unit 102in respective measurement cycles in which the optical delay unit isdriven according to the respective speed patterns.

The averaged data can further processed in processing unit 105 and theresult can be presented (step S408) by means of displaying or printing(outputting) in a known manner. For example, to present a frequencyspectrum, the averaged data of the time-domain waveform is subjected toa Fourier transform, and the frequency spectrum can be displayed orprinted for analysis. To present an image by moving a sample, an imageis produced by referring to coordinates at which data is measured. Thethus produced image can then be stored, transmitted or displayed asnecessary.

In the above-described manner, the apparatus and the method according tothe present embodiment of the invention enable accurate measurement ofthe time-domain waveform of the terahertz wave. In the presentembodiment, the speed pattern is changed to change the sampling timeinterval at which to acquire data for use in producing the time-domainwaveform of the terahertz wave. The resultant produced time-domainwaveform of the terahertz wave does not depend on the sampling timeinterval, but the change in the sampling time interval results in achange in shape of signal components with low correlation with theterahertz wave. This feature makes it possible to suppress the signalcomponents having low correlation with the terahertz wave by averagingdata obtained for various speed patterns. Thus, as shown in FIG. 3D, itis possible to obtain a time-domain waveform in which signal componentshaving less correlation with the terahertz wave are more suppressed thancan be achieved by the conventional technique. This makes it possible tosuppress a spurious spectrum superimposed on the frequency spectrum andthus it becomes possible to make a measurement for an increasedmeasurement frequency band.

The embodiments of the present invention may also be realized byperforming processes as described below. That is, software (a program)for realizing the functions of the embodiment is supplied to a system oran apparatus via a network or a storage medium, and the program is readand executed by a computer (a CPU, a MPU, or the like) in the system orthe apparatus. Note that any type of storage medium is usable as long asit is capable of storing the program in a form readable and executableby a computer, so that when the program is executed by the computer, themethod of measuring the terahertz wave can be practiced.

Next, a specific example is described below. In the description of theexample, similar parts to those described above are not explained again.In the apparatus shown in FIG. 1, the present example of the apparatusis configured as follows. The generating unit 101 and the detection unit102 are each realized using a photoconductive device including a galliumarsenide film grown at a low temperature and a pattern of an antennaelectrode formed on the gallium arsenide film. The laser light source108 is realized using a titanium-sapphire laser light source configuredto output laser light with a pulse width (FWHM) of 50 femtoseconds at arepetition frequency of 80 MHz. The optical delay unit 103 is configuredusing a one-stage folded optical system and a linear motion stage. Aretroreflector is used as the folded optical system. The linear motionstage is configured to be movable over a range of 14 mm (correspondingto an optical path length A needed in acquiring a time-domain waveform).The linear motion stage is driven according to speed patterns managed bythe driving speed adjustment unit 104. The driving speed adjustment unit104 and the processing unit 105 are both realized by a single arithmeticprocessing unit. In an operation of moving the linear motion stage, thedriving speed adjustment unit 104 controls the linear motion stage via astage driver. The processing unit 105 monitors the output of the currentdetection unit 107 and the position of the linear motion stage.

The current detection unit 107 includes a current-voltage conversionamplifier and a lowpass filter. In the present example, the conversionratio of the current-voltage conversion amplifier is set to 1×10⁸ (V/A).The lowpass filter is configured to have a cutoff frequency of 8 kHz.The bias applying unit 106 is realized using a low-noise DC voltagesource. In the present example, the bias applying unit 106 is configuredto supply a DC voltage of 30 v to the generating unit 101. In thepresent example, a time-domain waveform of a terahertz wave is producedwithout modulating the terahertz wave. For this reason, the timeconstant τ of the THz-TDS apparatus is given by the time constant of thecurrent detection unit 107. More specifically, the lowpass filter has atime constant of 0.125 msec (the reciprocal of the cutoff frequency),and the time constant τ of the THz-TDS apparatus is given by this timeconstant of the lowpass filter.

Referring to FIG. 7, the speed pattern used in the present example isdescribed below. In the present example, as shown in FIG. 7, the speedpattern is set such that the probability of occurrence is equal for anyvalue in the range centered at the average measurement time andincluding values from the minimum measurement time to the maximummeasurement time. More specifically, as shown in FIG. 5A, the speedpattern has a speed that changes at a constant speed in each time-domainwaveform measurement. In the present example, the number of times thetime-domain waveform is measured is set to 100.

The average speed Vave at which the optical delay unit 103 is driven isset to 7 mm/sec. Because the moving range A of the optical delay unit103 is equal to 14 mm, the average measurement time needed to measurethe time-domain waveform of a single terahertz wave is about 2 sec. Thetime-domain waveform has 4096 measurement points. The optical delay unit103 moves 3 μm between each adjacent measurement points. Because theoptical delay unit 103 employs the folded optical system, the amount ofchange X in optical path length between each adjacent measurement pointsis equal to 6 μm/point. As described earlier, X/τ/Vmax is set to beequal to or greater than 5. When parameters are set in theabove-described manner, the upper limit of Vmax is 9.6 mm/sec. In thepresent example, Vmax is set to this value. Note that Vmax may be smallthan this value depending on a situation. From the values of Vmax andVave, Vmin is given as 5.5 mm/sec. From these values, in FIG. 7, theaverage measurement time is 2 sec, the minimum measurement time is about1.45 sec, and the maximum measurement time is about 2.55 sec. Asdescribed above, the speed pattern is selected for each time-domainwaveform such that the measurement time falls within the above range.

A measurement result using the present example and a comparativemeasurement result are shown in FIG. 8B and FIG. 8A. Note that in eachof FIGS. 8A and 8B, only part of a time-domain waveform of a measuredterahertz wave is shown. More specifically, a part immediately before apulse waveform rises up is extracted from the total time-domain waveformof the terahertz wave and is shown. In these figures, each horizontalaxis represents a time axis of the time-domain waveform produced by theprocessing unit 105. Each vertical axis represents a current output fromthe detection unit 102 where the value of the current is calculated fromthe output of the current detection unit 107. FIG. 8A illustrates acomparative measurement result obtained when the optical delay unit 103is driving according to a fixed speed pattern. More specifically, thelinear motion stage of the optical delay unit 103 is driven at a fixedspeed equal to Vmax. FIG. 8B illustrates a measurement result obtainedin the present example. In the measurement, the speed pattern israndomly selected for each measurement of the time-domain waveformwithin a range from Vmin to Vmax.

It can be seen from comparison between FIG. 8A and FIG. 8B, there is adifference in the time-domain waveforms obtained. That is, in thetime-domain waveform (shown in FIG. 8A) measured using the fixed speedpattern, a signal is detected that has a maximum amplitude of ±0.015 nAand a period of about 1 psec. On the other hand, in the time-domainwaveform (shown in FIG. 8B) measured using the speed pattern varied foreach measurement, the periodic signal is suppressed. FIGS. 9A and 9Billustrate frequency spectra corresponding to the time-domain waveformsdescribed above. More specifically, FIG. 9A illustrates a frequencyspectrum corresponding to the time-domain waveform measured using thefixed speed pattern, and FIG. 9B illustrates a frequency spectrumcorresponding to the time-domain waveform measured using the speedpattern varied for each measurement. As can be seen from these twofigures, although two spurious spectrum components appear at 10 THz to11 THz in the spectrum obtained when the fixed speed pattern is used,the spectrum obtained when the speed pattern is changed for eachmeasurement has no spurious spectrum components and has a flat noisefloor.

As described above, by changing the speed pattern for each measurementof a time-domain waveform and averaging obtained data, it becomespossible to suppress a signal component having a low correlation withthe terahertz wave more effectively than can be by the conventionaltechnique. Thus, it becomes possible to suppress the spurious spectrumsuperimposed on the frequency spectrum and it becomes possible toincrease the measurement bandwidth.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

What is claimed is:
 1. A method of measuring a time-domain waveform of aterahertz wave based on time-domain spectroscopy by using an opticaldelay unit to adjust an optical path length along which excitation lightpropagates thereby adjusting a difference between a time at which theexcitation light arrives at a generating unit configured to generate theterahertz wave and a time at which the excitation light arrives at adetection unit configured to detect the terahertz wave, the methodcomprising: driving the optical delay unit according to a first speedpattern to acquire a first time-domain waveform; driving the opticaldelay unit according to a second speed pattern different from the firstspeed pattern to acquire a second time-domain waveform; and averagingthe first time-domain waveform and the second time-domain waveform.